High Entropy Oxides as Promising Materials for Thermal Barrier Topcoats: A Review

Two layered thermal barrier coatings (TBCs) are deposited on gas-turbine components to protect them against high temperatures, oxidation, and corrosion. However, the working temperature and lifetime of TBCs are limited due to material properties. To improve the life of TBCs, researchers are studying new approaches to increase phase stability and the properties of TBC topcoats. The concept of increasing entropy to stabilize phases was introduced in 2004, by decreasing Gibbs free energy. Different high entropy ceramics were introduced for structural and functional applications, while High Entropy Oxides (HEOs) were considered promising TBC ceramics due to their unique properties. HEOs are single-phase solid solutions that contain five or more cations, usually a mixture of transition metals and rare-earth elements. Due to the cocktail effect, the final material has different behavior from its constituents, which makes it a good method to improve the properties of traditional materials. Generally, for high entropy materials, three other phenomena affect the microstructure and properties, including high entropy, severe lattice distortion, and sluggish diffusion. In this paper, the use of HEOs as TBC topcoats is reviewed, also potential compositions, crystal structures, and thermophysical properties, as well as production methods, are studied.